The present invention relates to a hole drilling apparatus and, in particular to a collet for securing a drilling mechanism to a fixture which orients the drilling mechanism relative to a workpiece.
The drilling of holes in workpieces such as metal workpieces is usually performed by bringing the workpieces to a drilling machine. It sometimes occurs, however, that the workpiece cannot be conveniently brought to the drilling machine, e.g., the workpiece is too bulky, requiring that the drilling machine be brought to the workpiece. If the holes must be drilled to a higher degree of precision than can be obtained by a manual aligning of the machine, then drilling machines are used which can be clamped to a fixture or template disposed in front of the workpiece.
For example, shown in FIGS. 1 and 2 is a conventional drilling machine 10 manufactured by Cooper Power Tools and designated as a "P2 Drill with Concentric Collet Foot." The machine includes a front clamping unit having a housing 12 secured to the front of a drilling device 14, the device 14 carrying a drill bit (not shown). Projecting forwardly from the housing 12 is a hollow tubular nosepiece 16 that has a pair of recesses 18 formed in a front end thereof for discharging cuttings during a cutting action. An outer peripheral surface 20 of the nosepiece 16 is of slightly conical shape, i.e., it tapers forwardly, forming a small cone angle of 2.5.degree. for example. Slidably disposed on the nosepiece is a collet 22, shown also in FIGS. 3 and 4. The collet comprises a single piece of machined metal forming a tubular front clamping portion 24 and an integral rear displacement portion 26.
The clamping portion 24 is hollow and includes an inner periphery 28 having a forward taper corresponding to that of the outer surface 20 of the nosepiece. The outer periphery 30 of the clamping portion is of cylindrical shape, and longitudinal slits 32A, 32B are formed radially therethrough in circumferentially spaced relationship to enhance the elasticity of the clamping portion. None of the slits extends longitudinally completely through the clamping portion. That is, some of the slits 32A extend rearwardly from the front end of the clamping portion, but terminate short of a rear end thereof, whereas others of the slits 32B extend from the rear end of a bight portion 34 of the displacement portion 26, but terminate short of the front end of the collet.
The yoke-like displacement portion 26 includes a pair of legs 36 extending rearwardly from opposite sides of the bight 34. Those legs 36 are provided with mutually aligned holes 38 for receiving drive pins as will be explained.
When the collet 22, which is slidably mounted on the nosepiece 16, slides rearwardly, the clamping portion 24 is expanded radially outwardly, due to the 2.5.degree. taper. Conversely, when the collet is returned forwardly, the elastic clamping portion springs back to its original relaxed state.
Movement of the collet is effected by a pneumatic cylinder 40 having a piston rod 42 whose free end is affixed to a block 44. The block 44 is pivotably connected to lower ends of parallel links 46 whose upper ends are pivotably connected to opposite ends of a horizontal rod 48. Mounted to each link 46 intermediate the ends of the link is a horizontal drive pin 50 which extends into a respective one of the holes 38 formed in the collet 22. The pins 50 are slidable horizontally in respective horizontal slots 52 formed in the housing.
When it is desired to drill one or more holes in a workpiece 54 such as a metal workpiece (see FIG. 2), a fixture 56 in the form of a template 56 is secured immovably in front of the workpiece in a conventional manner. The fixture is provided with the same hole pattern to be formed in the workpiece. A small space 58 is left between the workpiece and fixture to accommodate the passage of cuttings during the drilling operation.
The nosepiece 16 and collet 22 are inserted through one of the holes 60 of the fixture 56 until the nosepiece contacts the workpiece 52. Then, the pneumatic cylinder 40 is actuated to retract the block 44, thereby causing the links 46 to pivot in a manner displacing the pins 50 rearwardly in the slots 52. The pins 50 pull the collet 22 rearwardly along the tapered outer surface of the nosepiece 16, causing the clamping portion 24 of the collet to be elastically expanded outwardly into tight clamping contact with the wall of the hole 60, thereby securing the drill mechanism 10 against movement with respect to the fixture 56 and thus with respect to the workpiece 52.
Then, the drilling device is actuated to extend a rotating drill bit (not shown) forwardly through the nosepiece 16 to drill a hole in the workpiece. If additional holes are to be drilled, the reverse procedure is followed to enable the collet to be retracted, whereafter the nosepiece and collet are inserted into the next hole of the fixture, and the above-described sequence of steps is repeated.
Among the drawbacks of the above-described apparatus is that the collet may sometimes break prematurely. That is, a clamp force of hundreds of pounds, e.g., 700-800 pounds, is typically applied to the collet, which force is transmitted through the two narrow legs 36 of the collet. Because of the high force and stress concentrations, premature breakage of the legs 36 may occasionally occur.
Furthermore, when it is subsequently attempted to release the collet from the hole 60 by displacing the collet forwardly along the nosepiece, strong resistance is encountered since the collet is tightly wedged between the nosepiece and the wall of the hole by the very slight 2.5.degree. taper of the nosepiece. In fact, the resistance may be so great as to require that a shoulder be placed against the rear of the drilling machine to prevent the drill device 14 from being displaced rearwardly while trying to displace the collet forwardly.